The plant’s operators and the regulators of the federal agency that oversees nuclear plants say these threats are likely to stay asleep.

And even if the Sanatoga fault, the Chalfont fault or the Ramapo fault awoke and shook those landmark towers in an earthquake, it’s unlikely that quake would be severe enough to cause enough damage to the plant to create a disaster.

That was the conclusion reached by geologists who mapped the local faults in 1974 for Philadelphia Electric Company.

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Despite improvements in geologists’ understanding of the East Coast earthquake picture — clearly punctuated by last August’s unexpected Virginia tremor felt in several states — that conclusion remains unshaken for both the plant’s current owner, Exelon Nuclear, and the Nuclear Regulatory Commission that oversees the plant.

Further, although newly revised earthquake risk assessments will be a factor in any new nuclear plants licensed by the NRC, they will not be considered when evaluating the 20-year license renewal for Exelon Nuclear’s Limerick Generating Station or any other nuclear plant for that matter.

In the end, it comes down to how risk is calculated.

“It is concluded that movement along the shears could not have occurred later than 500,000 years ago,” reads a July 30, 1974, letter signed by Joseph A. Fischer, a partner at Dames and Moore, and Bernard Archer, a senior geologist for the firm.

“In all probability” the faults have been inactive since 150 million to 200 million years ago, according to the letter that accompanied the green-shaded map showing the Sanatoga fault and Linfield fault zone close to a nuclear power plant.

But three independent earthquake experts said recently that they agreed with the Dames and Moore assessment of the earthquake risk being low, but they also all agreed that the operative word here is “probability.”

Not only has accurate earthquake prediction proven to be an elusive goal for modern science, even some of the methods used for gauging a fault’s risk of producing an earthquake – like those used near the Limerick site — may be ineffective.

In the 1970s, when looking for the potential earthquake risk a fault might pose, the standard test was to dig trenches across the fault line “to show that it hasn’t moved in 100,000 years,” said John Armbruster.

The problem with that test is, “some earthquakes occur at depth,” Armbruster said. “There may have been four or five earthquakes on a fault that don’t show any evidence they have moved.”

Asked if that invalidates the conclusions from the trench tests done in the 1970s, Armbruster paused and said, “I agree, it’s a contradiction” to conclude the absence of movement evidence means there have been no recent earthquakes.

Nevertheless, Armbruster said, although such testing methods are more accurate for active fault regions like California’s infamous San Andreas Fault, it is not entirely disproven as the basis for such a conclusion here in the east.

“To an extent, it’s a valid test for a fault that’s very close to a plant,” Armbruster said. “If it shows it hasn’t moved in 100,000 years, you probably won’t see it move again in the life of the plant.”

There’s that “probably” word again.

Eliza Richardson is an assistant professor with the Geo-sciences Department at Penn State who also reviewed the map. “I look at this and we could both live our entire lifetimes and never see any action in these faults,” she said.

She added, “I see a bunch of dotted lines and that usually means a ‘dike’ is assumed, but if all those things they’ve inferred are all part of the same fault, then that would be a pretty big fault going through there. If that’s true, it might be worth re-assessing,” Richardson said.

She noted that “now, with the equipment that we have, we could certainly get a clearer picture of what’s down there, but it wouldn’t be free.”

And last year, when MSNBC.com analyzed the new NRC assessments, it ranked the Exelon Nuclear’s Limerick Generating Station as the plant whose risk of earthquake damage had risen by the third highest amount.

Chances of an earthquake damaging a plant here were raised by 141 percent, from 1 in 45,455 to 1 in 18,868, by MSNBC.com.

True, you’d still have a better chance of winning the lottery than seeing the towers topple in an earthquake, but the increased risk has not gone unnoticed by the NRC.

Diane Screnci, senior public affairs officer for the NRC’s Region 1 office in King of Prussia, said the NRC has been reviewing earthquake risk to plants since 1988.

“Currently, the NRC is in the process of conducting a generic review to again assess the resistance of U.S. nuclear plants to earthquakes,” she wrote in a May 15 e-mail.

The review gained a new urgency in the wake of the Japanese earthquake and nuclear disaster.

A preliminary analysis of the data collected so far put the “average probability” of an earthquake being stronger than an east coast plant’s ability to withstand at “less than 1 percent,” Screnci wrote.

“The station was designed to withstand the largest local seismic activity in recorded history plus additional margin,” wrote Melia, who added, “in fact, if a major earthquake were to strike our region, Limerick Generating Station is one of the safest places to be.”

As it turns out, both those elements — earthquake predictability and ability of a nuclear plant to withstand an earthquake greater than the experts thought possible — came into play Aug. 23 when an earthquake outside of Richmond, Va. shut down the North Anna nuclear plant.

“The earthquake in Virginia last year had shaking that was way more than was expected and that plant and reactor were shaken more than it was designed to withstand,” said Armbruster, the Earth Observatory seismologist. “Fortunately, it was over-designed.”

But although the NRC and nuclear power plant operators would argue that demonstrates the concept of over-design works, the North Anna plant, operated by Dominion Virginia Power, did not escape unscathed.

In the wake of the quake, NRC inspectors concluded the plant operators had violated NRC rules by failing to adequately maintain the plant’s emergency diesel generators, one of which failed after the Aug. 23 quake.

It was the failure of emergency diesel generators at the Fukushima Daiichi plant in Japan, and the resultant failure to continue to pump water into the plant to keep the radioactive fuel cool, which ultimately caused the nuclear disaster there.

That was not the case, apparently, in the late 1970s when the North Anna plant was being built, at nearly the same time and over a similar fault, as Limerick.

According to a Nov. 1, 2011 letter from the head of the Project on Government Oversight, a non-profit advocacy group, the utility company building the plant concealed the discovery of the fault under the site while the North Anna plant was under construction. The letter also said the NRC was complicit in doing little to require improvements in design at that plant, or to inform the public of the discovery.

That conclusion was shared by the U.S. Department of Justice, which launched a criminal investigation into the matter in 1977.

The department concluded it could not be successful prosecuting the utility in court largely due to “the actions of the NRC itself, which, in the best light, can be characterized as ill-considered and inept and perhaps more realistically, as demonstrating a pervasive bias against public scrutiny.”

That was not the case when Limerick was built, according to Screnci.

She pointed to a 1983 document, not available on-line, which “noted that three faults have been mapped and investigated within two miles of the (Limerick) site, including the Sanatoga fault. A panel of experts in Appalachian geology and NRC staff concluded that these faults have experienced their last displacements more than 500,000 years ago.”

They concluded “there were no capable faults in the site area.”

Ten years before that Limerick report, the Virginia utility ultimately succumbed to public pressure and Congressional outrage and submitted a report to NRC which similarly concluded the fault under that plant “was older than 1 million years and did not pose a risk,” according to the POGO letter.

Thirty-eight years later, that one million-year-old fault was at the epicenter of the largest earthquake east of the Rocky Mountains since 1897.

Richardson, the Penn State expert, said despite all the expert opinion, it’s not too surprising that the assessment of the Virginia fault was off, pointing to the Aug. 22 earthquake as evidence.

“The study of earthquake dynamics is hard,” said Richardson. “It’s not like Kepler’s laws of planetary motion where I can tell you what time the moon will rise and where.”

It’s relevant to note that earthquake experts were surprised by the power of the March, 2011 earthquake off the Japanese coast which ultimately caused the Fukushima nuclear plant disaster. The Japanese earthquake originated from a fault experts did not consider particularly active or capable of such force.

So while earthquake expertise has value, it is not a sinecure, either in Virginia or, possible, in Limerick, she said.

Referring to the Dames and Moore map, Richardson said “the faults on this map are roughly similar to the one that caused the northern Virginia earthquake. One earthquake of that size does not happen every day, there are 100 places more likely that you would expect to see, but that one happened there where no one expected it,” she said.

But it will not, Screnci said, be used when considering re-licensing of existing plants, including Exelon’s application to renew its two Limerick reactors for an additional 20 years.

“License renewal focuses on whether the plant has programs and processes to effectively manage the effects of aging in a period of extended operation,” Screnci wrote.

“The commission considered whether or not to include plant responses to external events that may be outside the licensing basis, but reasoned that the existing regulatory process was sufficient to address those instances while at the same time avoiding duplicative and, perhaps, less efficient assessments,” she wrote.

So although new earthquake data and risk models will be used for licensing any new plants built in the U.S., “the commission maintained that the focus of license renewals” for older plants that were built when the risk from earthquakes was presumed to be lower “should be limited to the age-related degradation” for plant systems and components.

About the Author

Evan Brandt has worked for The Mercury since November 1997. His beat includes Pottstown, the surrounding townships and the Pottstown and Pottsgrove school districts, as well as other varied general topics like politics, the environment and education. Reach the author at ebrandt@pottsmerc.com
or follow Evan on Twitter: @PottstownNews.